There are many known welding processes, used in many different applications. One well known welding applications is controlled short circuit welding, which generally consists of alternating between an arc state and a short circuit, non-arc state. During the arc state the wire melts, and during the short circuit state the metal further melts and the molten metal is transferred from the end of the wire to the weld puddle. The metal transferred in one cycle is referred to herein as a drop, regardless of the size or shape of the portion of metal that is transferred. This process has many advantages, such as shorter arc length and less melting of the base plate.
Controlled short circuit welding includes waveform controlled short circuit welding and mechanically controlled short circuit welding. Waveform controlled short circuit welding controls the process by controlling the output voltage and current (the power waveform) to have desired values, such as those which will reduce spatter by reducing the pinch force (current) when the liquid-metal bridge of the short is about to break and establish an arc. Mechanically controlled short circuit welding controls the process by controlling the breaking of the short by retracting the wire (mechanically controlling the wire and process) and can reduce the current and/or voltage to a desired level before the wire is retracted.
Both the power source topology and the control scheme must be considered when designing a waveform controlled short circuit welding power supply. The power topology used must be fast enough to have a timely response to the chosen control scheme. The control should address three considerations: First, arc length must be properly controlled. Second, the burn-off (or mass deposition) rate must be appropriately controlled. Inappropriate burn-off rate will result in increased spatter. Third, spatter is also caused by too much power when the short is cleared, i.e., the transition from a short circuit to an arc. Thus, the power or current when the short clears must also be controlled. Also, when the short is about to clear must be detected (for waveform controlled short circuit welding).
Known waveform controlled short circuit welders often have a converter or inverter controlled to provide the desired output. Generally, they lower the current prior to the short clearing so that the current is low when the short clears and the arc forms. The current is lowered because the clearing can be an explosive event, and lower current reduces spatter. Such systems attempt to sense or predict the short ending, and lower the current in response thereto. For example, U.S. Pat. Nos. 6,087,626 and 6,326,591 to the present inventor (hereby incorporated by reference), describe a topology and control that are used for short arc welding. They predict the short clearing, and reduce the inverter output before the short clears.
Other waveform controlled short circuit welders include an output bypass switch that shunts a resistance between the power circuitry and the weld. This switch is used to lower the current faster than commanding the inverter (or other power circuit) can lower the current. An example of such a system is found in U.S. Pat. No. 5,001,326 (hereby incorporated by reference), as well as in a number of related patents. Switch 70 of that patent is referred to herein as an output bypass switch because it is on the output of the inverter, and allows the output current to be more quickly reduced than it would be by control of the power circuitry alone. Power circuits used with this sort of system include inverters and dc supplies.
Prior art control schemes typically allow the user to set the peak and background current, and the control scheme controls the output accordingly. While the user can adjust peak and background current, the user cannot adjust the heat content of the drops. To the extent the heat content is controlled, it is fixed by the particular control scheme. Some welds could be better performed if the heat could be adjusted. For example, more heat might be desired if the user determines the puddle has not properly wetted out. Or, less heat might be desired if there is too much penetration. Unfortunately, prior art control schemes only allow the user to indirectly adjust the heat content by adjusting current magnitudes.
Accordingly, a controlled short circuit welding system that allows the user to adjust the heat content of the drops is desirable. Preferably, a baseline heat content is established by the control scheme, and the user can increase or decrease the heat content.